Light Year :


unit of length used in astronomy to measure vast distances. It is equal to the distance that light travels in a mean solar or astronomical year. At the rate of approximately 300,000 km/s (186,000 mi/s), a light year is approximately equal to 9,461,000,000,000 km (5,880,000,000,000 mi).



Parsec:
unit of measurement used, along with the light year, for stellar distances. One parsec is the distance to a star that shows a parallax, or apparent shift in position when viewed from opposite sides of the Earth's orbit, of two seconds of arc. (The word parsec is an abridgment of the words parallax and second.) This distance is 30.86 million million km (19 million million mi). One parsec is equal to 3.26 light years and to 206,265 astronomical units.